1. Field of the Invention
The present invention relates to a method and apparatus for removing vapor phase contaminants from a gas stream. More particularly, the present invention relates to a method and apparatus for the removal of vapor phase contaminants from flue gases generated by a coal-fired boiler using a membrane.
2. Description of Related Art
Utility power plants are concerned about emissions of trace metals in light of the 1990 Clean Air Act Amendment (CAAA) on air toxics (Title III). Special attention has been given to mercury (Hg) in terms of its environmental release and impacts, and the Environmental Protection Agency (EPA) is closely scrutinizing sources that emit mercury. The EPA has determined that utility power plants, and specifically coal-fired power plants, are the major remaining source of mercury emissions into the air. Mercury is present in flue gas generated by coal-fired power plants in very low concentrations (<5 ppb) and forms a number of volatile compounds that are difficult to remove. Specially designed and costly emissions-control systems are required to capture these trace amounts of volatile compounds effectively.
Several approaches have been adopted for removing mercury from gas streams. The most common methods are often called “fixed-bed” techniques. In these systems, the gas containing mercury is passed through a bed consisting of sorbent particles held in place by various structures such as honeycombs, screens or fibers. A common sorbent is activated carbon in powder form.
There are, however, several disadvantages of fixed bed systems. Gas streams such as those from power plant coal combustion contain significant fly ash that can plug the bed structures and, thus, the beds need to be removed frequently from operation for cleaning. Alternatively, these beds may be located downstream of a separate particulate collector (see, for example, U.S. Pat. No. 5,409,522, entitled “Mercury Removal Apparatus and Method,” which is incorporated by reference herein in its entirety). Particulate removal devices ensure that components of the flue gas such as fly ash are removed before the gas passes over the mercury removal device. The beds will still need to be taken off-line periodically for regeneration, thereby necessitating a second bed that can remain on-line while the first one is regenerating. These beds also require significant space and are very difficult to retrofit into existing systems such as into the ductwork of power plants without major modifications.
In another process for removing mercury or other vapor phase contaminants from a flue gas stream, a carbonaceous starting material is injected into a gas duct upstream of a particulate collection device. The carbonaceous starting material is activated in-situ and adsorbs the contaminants. The activated material having the adsorbed contaminants is then collected in a particulate collection device. Such a process is described in U.S. Pat. Nos. 6,451,094 and 6,558,454, both entitled “Method for Removal of Vapor Phase Contaminants From a Gas Stream by In-Situ Activation of Carbon-Based Sorbents,” which are both incorporated by reference herein in their entireties. However, the need to replenish the carbonaceous starting material and collect the spent activated material in a particulate collection device creates extra steps that consume additional resources.
In yet another process to remove mercury in a flue gas stream, gold and/or metals from Groups IA, IB, and III of the Periodic Table of Elements can be used as a sorbent to adsorb mercury, as described in U.S. Pat. No. 5,409,522, entitled “Mercury Removal Apparatus and Method,” which is incorporated by reference herein in its entirety. In this process, mercury is adsorbed and amalgamated with the sorbent that is disposed on a collection surface. The sorbent can be regenerated by applying heat to the sorbent surface, thereby releasing the mercury-containing compound. Even though the sorbent can be regenerated to save costs, the regeneration process creates an extra step that consumes time and other resources to set up and maintain.
Elemental mercury is particularly difficult to remove by these conventional methods. To address this difficulty, gold and/or other noble metals may be used as a catalyst to convert elemental mercury (Hg(0)) into an oxidized form, such as mercury (II) chloride, since oxidized forms are easier to remove. Oxidation of mercury through gold and/or other noble metals is possible when handling flue gas streams containing at least 20 ppm HCl gas. The gold and/or noble metal may be disposed on a catalyst bed supported by a screen in a duct. Such a process is described in U.S. Pat. No. 6,136,281, entitled “Method to Control Mercury Emissions from Exhaust Gases,” which is incorporated by reference herein in its entirety.
In view of the foregoing, there exists a need for an improved method and apparatus for removing vapor phase contaminants such as mercury from a gas stream.